A current conventional vehicle wheel alignment system uses sensors or heads that are attached to the wheels of a vehicle to measure various angles of the wheels and suspension. These angles are communicated to a host system, where they are used in the calculation of vehicle alignment angles. In the standard conventional aligner configuration, four alignment heads are attached to the wheels of a vehicle, Each sensor head comprises two horizontal or toe measurement sensors and two vertical or camber/pitch sensors. Each sensor head also contains electronics to support overall sensor data acquisition as well as communications with the aligner console, local user input, and local display for status feedback, diagnostics and calibration support.
In recent years, wheels of motor vehicles have been aligned in some shops using a computer-aided, three-dimensional (3D) machine vision alignment system. In such a system, one or more cameras view targets attached to the wheels of the vehicle, and a computer in the alignment system analyzes the images of the targets to determine wheel position and alignment of the vehicle wheels from the wheel position data. The computer typically guides an operator to properly adjust the wheels for precise alignment, based on calculations obtained from processing of the image data. A wheel alignment system or aligner of this image processing type is sometimes called a “3D aligner.” Examples of methods and apparatus involving computerized image processing for alignment of motor vehicles are described in U.S. Pat. No. 5,943,783 entitled “Method and apparatus for determining the alignment of motor vehicle wheels;” U.S. Pat. No. 5,809,658 entitled “Method and apparatus for calibrating cameras used in the alignment of motor vehicle wheels;” U.S. Pat. No. 5,724,743 entitled “Method and apparatus for determining the alignment of motor vehicle wheels;” and U.S. Pat. No. 5,535,522 entitled “Method and apparatus for determining the alignment of motor vehicle wheels.” A wheel alignment system of the type described in these references is sometimes called a “3D aligner” or “visual aligner.” An example of a commercial vehicle wheel aligner is the Visualiner 3D, commercially available from John Bean Company of Conway, Ark., a unit of Snap-on Inc.
Alternatively, a so-called “hybrid” machine vision wheel alignment system may include a pair of passive heads and a pair of active sensing heads. The passive heads are for mounting on a first pair of wheels of a vehicle to be measured, and the active sensing heads are for mounting on a second pair of wheels of the vehicle. Each passive head includes a target, and each active sensing head includes gravity gauges for measuring caster and camber, and an image sensor for producing image data, including an image of a target of one of the passive heads, when the various heads are mounted on the respective wheels of the vehicle. The system also includes a spatial relationship sensor associated with at least one of the active sensing heads, to enable measurement of the spatial relationship between the active sensing heads when the active sensing heads are mounted on wheels of the vehicle. The system further includes a computer for processing the image data relating to observation of the targets, as well as positional data from the spatial relationship sensor, for computation of at least one measurement of the vehicle.
Conventional methods of adjusting the individual toe angles of the front wheels of a vehicle require leveling and locking the vehicle's steering wheel. This method applies to hydro-mechanical power steering systems which do not have electronic sensors mounted on the steering column, as well as “servotronic” types of systems. Such hydro-mechanical power steering systems depend on hydraulic fluid pressure to provide steering assist, typically provided by an engine-driven hydraulic pump.
Many newer vehicles are equipped with electro-mechanical power steering (EPS) systems which use an electric motor to provide steering assist. The electric motor is controlled by an electronic control module (ECM), which receives signals from electronic sensors such as a steering wheel angle sensor, a torque sensor, a summation sensor, and a vehicle speed sensor. A typical EPS system has a torque sensor, torsion bar, and steering angle sensor mounted on the steering column and/or steering gear to track the position of the steering wheel and allow the ECM to respond to a torque request for power assist.
Unlike hydro-mechanical systems, EPS systems are sensitive to steering rack movement during alignment adjustment. Any movement in the steering rack imposes rotation on the steering shaft, rotating the steering angle sensor. If the steering wheel is held in place at a level position using a steering wheel holder (as in conventional alignment procedures), the torque sensor cannot respond to the imposed rack movement. Consequently, a lack of synchronization or “delta” is created between the steering angle sensor and torque sensor. The delta is unknown to the alignment technician. However, the ECM will recognize this delta and interpret it as a request for torque assist (i.e., power steering) and will send a command to the electric motor to provide power assist that will steer the vehicle, resulting in the driver of the vehicle disadvantageously perceiving a “pull” normally associated with improper wheel alignment.
There exists a need for an alignment apparatus and method for EPS-equipped vehicles that does not result in a lack of synchronization between EPS sensors, and thus does not require re-synchronization of the sensors after alignment.